cpp - Process technology for the chemical industry 02.2018

cpp ACHEMA PROCESS ENGINEERING
Pictures: SGL Carbon
Liquid distributor made of carbon fibre composites for hydrochloric acid at temperatures greater than 150 °C
Hydrogen chloride recovery system for weak acid
Increased demand for
efficient processes
SGL Carbon is designing an HCl recovery system to recover and purify HCl
gas from a contaminated weak acid based on the pressure swing distillation
concept. This process produces pressurised HCl gas free of impurities. New
materials and an integrated heat recovery concept promise economical
operation and high reliability.
The customer is a leading manufacturer of
refrigerants, fluoropolymers and siliconbased
products in China. Methyl chloride is
an intermediate in the production of silicon.
It is obtained by reacting methanol
and hydrogen chloride. Hydrogen chloride
is typically produced by distilling concentrated
hydrochloric acid. Due to the corrosive
properties of hydrochloric acid, corrosion-resistant
materials such as graphite
and PTFE-lined steel are required for such
applications.
Increasing costs for the disposal of weak
acid are forcing manufacturers to use HCl
recovery systems to produce hydrogen
chloride. For this purpose, SGL Carbon is
implementing a customised system in
China that almost fully recovers hydrogen
chloride from contaminated acid.
Process options
Extractive rectification is typically applied
for the complete separation of hydrochloric
acid. Additives such as calcium chloride or
sulphuric acid are used to selectively increase
the volatility of hydrogen chloride in
order to break the azeotrope. One disadvantage
of this process is that the addition of
the entrainer results in a higher boiling
point in the rectification column. Due to
the limited temperature resistance of the
materials employed, only a lower HCl gas
pressure can be achieved. Consequently, the
subsequent processing of the hydrogen
chloride either requires elaborate compressors
or the lifetime of the equipment is
reduced. Since such processes can only be
operated efficiently with a high salt content,
there is a risk of crystallisation when
using alkaline salts as the extractive compound.
Additionally, impurities such as F - ,
(SO 4
) 2- , etc. may form salts that result in
severe scaling, thus endangering the reliability
of the complete system.
Pressure swing distillation systems avoid
80 cpp 02-2018